Assuming you are back to where you started..
- Mac1 is 450/500 GB used, new mac 13" late 2014
- Mac2 is 160/500 GB used, old mac 15" mid 2012
Method 1.
Make a clean TM backup of each computer. I strongly recommend you get a 1TB or larger USB and partition it.. so you have two 500GB at least and then make the clean TM backup of Mac1 to partition1 and Mac2 to partition 2. leave your existing TM backup on the TC alone. That is your failsafe.
(I would also create clones of each computer using Carbon Copy Cloner which you can run free for a month.. so if you get into deep trouble you can simply restore the clone).
Then clean install the OS on both laptops. Once that is finished.. migrate the whole lot from its opposite number using Time Machine.
This is the best way I can think of that gives you all the right drivers and clean installs you need on both computers.
I strongly recommend you buy a software .. carbon copy cloner. It is not expensive and you don't even have to pay for the license to do what you need. It has one month free trial if you want.
Method 2.
CCC has the ability to make a bootable clone. So simply clone Mac1 Once you are finished.. It will take a few hours.. not too long being USB3 and of course you will buy a USB3 disk.. test boot Mac 2 from the clone instead of the internal disk. This may or may not work.. but I have tested it and within a certain range of releases it will work.. I think 2012 to 2014 it is worth a go.. if it works then you can simply copy the clone to the Mac2 hard disk same as you copied to the clone to the USB drive. For Mac1 do the clean install as suggested and migrate the files from Mac2 TM backup.
Think carefully and make sure you have multiple methods of getting back to where you started from.
If Macs were built with removable hard disks as all other laptops in the world have you could have just swapped drives.. but you have effectively done the same thing.
